The present invention relates to the preparation of a low polydispersity index water-soluble polymeric composition. Water soluble polymeric compositions such as low molecular weight polyacrylates are widely used, for example, in detergent formulations as crystal growth inhibitors, dispersants, sequestration agents, antiscalants in water treatment processes, and as dispersants in clay and mineral processing as well as in coating formulations.
Water-soluble polymeric compositions that are produced by conventional free radical polymerization methods typically have a high polydispersity index (defined by weight average molecular weight over number average molecular weight or M.sub.w /M.sub.n). Although the theoretical limit for polydispersity (M.sub.w /M.sub.n) in radical chain polymerizations is 1.5 or less, in practice the distribution is greater than 2.
The performance of water-soluble polymers can depend strongly on both their molecular weight and M.sub.w /M.sub.n. For example, detergent performance is known to depend strongly on polymer molecular weight and structure. In conventional detergents, under low temperature U.S. wash conditions, clay removal is maximized and calcium carbonate fabric encrustation is minimized when detergents are formulated using sodium polyacrylate of approximately 5000 molecular weight. Higher molecular weight polymers, which are less effective crystal growth inhibitors and clay dispersants, allow calcium carbonate deposition to increase, resulting in reduced clay cleaning performance. Thus, there is an obvious need for controlled molecular weight and low polydispersity for acrylate polymers. See Witiak in Detergents and Cleaners: A Handbook for Formulators, ed., K. Robert Lange, Hanser Publishers, Munich, Vienna, New York, pp. 113-132 (1994); and Zini, "Polymeric Additives for High Performing Detergents" (1995).
In another example of the desirability of low polydisperse polyacrylates, Farrar et al. (U.S. Pat. No. 4,507,422, which teachings are incorporated herein by reference) discloses that water-soluble polymeric dispersants have greatly improved pigment dispersing properties if the polymer has a M.sub.w /M.sub.n of less than 1.5. It is presumed that fractions of the polymer with molecular weights that are too low or too high interfere with the function of the optimal molecular weight fractions by competitive absorption or dilution effects.
Farrar et al. teaches that water-soluble polyacrylates having a low M.sub.w (that is, a weight average molecular weight of about 1000 to 10,000) and low M.sub.w /M.sub.n can be achieved by a variety of techniques including: a) using conventional polymerization methods to obtain a polymer having high polydispersity, then fractionally precipitating the polymer to obtain polymers having a polydispersity below 1.5; b) synthesizing the polymer in the presence of isopropanol as a chain regulator under closely monitored and controlled conditions; and c) preparing a water-insoluble acrylate polymer having the desired molecular weight and M.sub.w /M.sub.n, then hydrolyze the acrylate to the free acid. However, all of these approaches are labor intensive and inefficient.
In U.S. Pat. No. 5,412,047, incorporated herein by reference, Georges et al. discloses the use of the carbonyl-containing nitroxide 4-oxo-TEMPO as a free radical agent suitable for controlling the molecular weight of acrylate polymers. Georges et al. further teaches (column 13, lines 65-68 to column 14, lines 1-4) that nitroxide compounds that do not contain oxo groups, while satisfactory for the purpose of moderating the polymerization of a wide variety of different monomer types, "were completely ineffective when used in homopolymerizations of acrylate monomers (for example, n-butylacrylate.)" Presumably, the homopolymer (column 14, lines 42-45) "was sufficiently thermally unstable or the stable free radical had a sufficiently strong inhibitory effect under the reaction conditions so as to preclude homoacrylate polymer product formation."
The reaction conditions disclosed by Georges et al. are in the range of 100.degree. C. and 180.degree. C.; at temperatures below 100.degree. C. (column 16, lines 13-16), "the reaction rate is slow and industrially impractical without the aid of an acid or base accelerating additive compound." At these elevated temperatures, the polymerization reactor must be equipped to operate at elevated pressure inasmuch as water is used as the solvent.
Another method for preparing low molecular weight low polydisperse acid polymers is to oxidatively degrade a soluble higher molecular weight polymer to a lower molecular weight lower polydisperse polymer, as described in EP 0404377 A1, which teachings are incorporated herein by reference. According to this publication, M.sub.w as low as 13000 and M.sub.w /M.sub.n as low as 2.1 can be achieved by oxidation of a high molecular weight, partially neutralized acrylic acid homopolymer with hydrogen peroxide.
In view of the deficiencies in the art, it would be desirable to prepare more efficiently and under milder conditions a water-soluble polymer having controlled molecular weight and low polydispersity.